Cryogenic devices

ABSTRACT

1,075,316. Refrigerating. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. Feb. 4, 1966 [Feb. 24, 1965], No. 8035/65. Heading F4H. A cryogenic device comprises a vessel 1 having a cold surface 2 and containing a cryogenic liquid at a predetermined temperature, an intermediate vessel 10 in communication with vessel 1 through duct 11, a connection 16 for supplying cryogenic liquid from a dewar vessel 3 to the vessel 10 and a valve 12 in the latter for opening the duct 11 only when the temperature of the liquid in vessel 10 is substantially the same as that of the liquid in vessel 1. Vessels 10, 1 and 3 have respective vapour outlets 14, 15, 23 which are each connectable to a common vapour pump (20, Fig. 2, not shown), via a set of three valves (VT1-3, VC1-3 or VD1-3) of progressively decreasing aperture, the valves being controlled by relays energized in response to signals from contacts disposed at different levels in the legs of manometers (Fig. 3, not shown) connected to the respective vessels. Thermometers T 1  and T 2  define the maximum and minimum level of liquid in vessel 1 and a thermometer T 3  is provided to detect the presence of a predetermined level of liquid in vessel 10. The pressures and temperatures within the vessels, the transfer of liquid from vessel 3 to vessel 10 and the transfer of liquid from vessel 10 to vessel 1 are controlled automatically by a logic circuit (Fig. 5, not shown) in response to signals from the pressure and temperature-sensing means. Surface 2 preferably comprises a condensing surface of a cryopump and vessel 1 is surrounded by a heat shield 7 which is an extension of a double-walled lliquid nitrogen containing vessel 8. The cryogenic liquid is preferably helium.

NV. 21, 1967 J, N, CHUBB 3,353,365

CRYOGENIC DEVICES Filed Feb. 9, 1966 l 5 SheeS-Sheet l Nov. 21, 1967 J.N. CHUBB 3,353,365

' CRYOGENIC DEVICES Filed Feb. e, 196e 5 sheets-sheet 2.

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Nov. 21, 1967 Filed Feb. 9, 1966 J. N. CHUBB CRYOGENIC DEVICES 3Sheets-Sheet 5 United States Patent O 3,353,365 CRYOGENIC DEVICES JohnNorman Chubb, Didcot, England, assignor to United Kingdom Atomic EnergyAuthority, London, England Filed Feb. 9, 1966, Ser. No. 526,125 Claimspriority, application Gt Britain, Feb. 24, 1965,

H 7 Claims. (Cl. 62--55.5)

ABSTRACT F THE DISCLGSURE A cryogenic device comprising a vesselcontaining cryogenic liquid which is maintained at a desired temperatureby vapour pumping. Provision is made for automatically replenishing thevessel with further cryogenic liquid as necessary, without causing anyappreciable fluctuation in the temperature of the cryogenic liquid inthe vessel. The device may form part of a cryopump.

This invention relates to cryogenic devices and particularly, but notexclusively, to cryopumps.

A cryopump is a form of vacuum pump which includes a cryogenicallycooled surface in contact with the gas within the system to be pumped.Gas molecules striking the cooled surface lose a part of their incidentenergy, and if the cooled surface is maintained lat a temperaturesuiciently below the normal boiling point of the gas in the system, thengas phase molecules are removed from the system by condensation on thecooled surface. For effective pumping the incident molecules shouldpreferably have a high sticking coefficient, that is to say there shouldbe a high probability that a gas molecule striking the cooled surfacewill condense, and the rate of evaporation must be lower than the rateof condensation.

To maintain an optimum balance between pumping etliciency andrefrigeration requirements it is desirable to keep the Whole condensingsurface Within close temperature limits. To achieve this over anextensive cooled surface and to provide good temperature stabilityagainst iluctuating heat loads it is desirable that the surface becooled by direct contact with a cryogenic liquid on its reverse side,rather than by contact with a gaseous refrigerant.

In a practical arrangement the cooled surface forms one wall of a vesselwhich contains la suitable cryogenic liquid, for example liquid helium,this wall projecting into the system to be pumped. The temperature ofthe cooled surface can be controlled by regulating the gas phasepressure over the cryogenic liquid. To allow operating times greaterthan can conveniently be achieved by storing cryogenic liquid within thevessel itself, it is necessary to replenish the cryogenic liquid fromtime to time.

Hitherto no satisfactory arrangement has been devised for accuratelymaintaining the temperature of the cooled surface Whilst the cryogenicliquid is being replenished, and one object of the present invention isto provide a cryopump including such an arrangement.

According to one aspect of the present invention, a cryogenic devicecomprises a first vessel arranged to contain a cryogenic liquid, meansto maintain the cryogenic liquid in the first vessel substantially at adesired temperature, a subsidiary vessel communicating directly with thelirst vessel, a connection through which cryogenic liquid is arranged tobe supplied from a second vessel containing a further quantity of thesame cryogenic liquid to the subsidiary vessel when the cryogenic liquidin the rst vessel needs replenishing, and means to allow cryogenicliquid to pass from the subsidiary vessel into the lirst vessel onlywhen the temperature of the cryogenic liquid in the subsidiary vessel issubstantially equal to said desired temperature.

According to another aspect of the present invention, a cryopumpcomprises a surface arranged to be in contact with the gas within asystem to be pumped, a first Vessel arranged to contain a cryogenicliquid in good thermal contact with said surface so that the surface ismaintained substantially at the temperature of the cryogenic liquid,this temperature being such that there is a net condensation of gasphase molecules from the system on to said surface, means to maintainthe cryogenic liquid in the irst vessel substantially at a desiredtemperature, a subsidiary Vessel communicating directly with the firstvessel, a connection through which cryogenic liquid is arranged to besupplied from a second vessel containing a further quantity of the samecryogenic liquid to the subsidiary vessel when the cryogenic liquid inthe rst vessel needs replenishing, and means to allow cryogenic liquidto pass from the subsidiary vessel into the first vessel only when thetemperature of the cryogenic liquid in the subsidiary vessel issubstantially equal to said desired temperature.

The invention may for example comprise a cryopump in which the cryogenicliquid is liquid helium and in which the gas to be evacuated from theSystem to be pumped is principally hydrogen. To maintain a useful vacuumin hydrogen it is desirable to work with said surface at a temperaturein the range 3.1 to 3.9 K. which corresponds to the temperature obtainedby boiling liquid helium under a reduced pressure in the range 210 to580 millimetres of mercury.

A cryopump in accordance With the present invention Will now bedescribed by way of example with reference to the accompanying drawings,in which:

FIGURE 1 shows a section through a part of the cryopump,

FIGURE 2 shows a part of the cryopump control system diagrammatically,

FIGURE 3 shows another part of the cryopump control systemdiagrammatically,

FIGURE 4 shows a partial section through another part of the cryopumpcontrol system, and

FIGURE 5 shows the control circuit of the cryopump diagrammatically.

Referring first to FIGURES l and 2 of the drawings, the cryopump to bedescribed is for use in maintaining the gas pressure in a system at adesired low value. The cryopump would normally be used after an ordinaryVacuum pump had reduced the pressure in the system to about 10-3millimetres of mercury. The cryopump includes a vessel l having a metalsurface 2 which is arranged to project into the system to be pumped. Thevessel 1 contains liquid helium which is replenished as necessary from aDewar 3.

The surface 2 is of the order of 60 square centimetres in area and is indirect contact and hence in good thermal contact with the liquid heliumin the vessel 1. Gas molecules striking the surface 2 lose a part oftheir incident energy and it is arranged that the surface 2 ismaintained at a temperature suiiciently below the normal boiling pointof the gas in the system for gas phase molecules to be removed from thesystem by condensation on the surface Z, For effective pumping theincident molecules must have a high sticking coelicient, that is to saythere must be a high probability that a gas molecule striking thesurface 2 will condense, and the rate of evaporation from the surface 2must be lower than the rate of condensation.

The particular embodiment of the cryopump being described is for use inassociation with experiments relating to plasma physics and controlledthermonuclear reactions and the gas to be pumped from the system isprimarily lydrogen. In this case it is preferable that the surface 2 mdhence the liquid helium in the vessel 1, should be naintained at sometemperature in the range 3.l K. to 9 K. to within i0.05 K., in orderthat the required ystem. pressure bel maintained and at the same timethe :ryopump should be operating eciently. The liquid lelinm in thevessel 1 is maintained at a temperature beow the normal boiling point ofliquid helium by coninuously pumping helium gas from the vessel I, sothat v11e pressure is maintained in the range of 210 to 580 nillimetresof mercury, which corresponds to liquid lelium temperatures in the range3.l K. to 3.9 K.

The cryopump therefore includes provision for ac- :urately maintainingthe temperature of the liquid helium n the vessel I at the desiredvalue, for replenishing the iquid helium in the vessel 1 when necessary,and for en- ;uring that there is no appreciable uctuation from the:lesired temperature during replenishment. The cryopump also includesvarious other arrangements for control, safety and for providingindications.

The various parts of the cryopump will now be described in more detail.

First the vessel I will be described in more detail with particularreference to FIGURE 1. The surface 2 is at the bottom of a wide portionof the vessel I, hereinafter referred to as the condenser 4, from whichextends a support tube 5 of low thermal conductivity, which passes outthrough a wall of the system to be pumped by way of a mounting flange 6.Surrounding the condenser 4 is a radiation shield 7y which is anextension of a doublewalled jacket 8 surrounding the lower part of thesupport tube 5, The jacket 8 is partly lled with liquid nitrogen whichis replenished periodically by way of a tube 9 which extends through theflange 6. The jacket 8 may advantageously be extended to join thesupport tube 5 near the flange 6, so as to intercept conducted heat flowin the support tube 5 from that part of it which is beyond the flange 6and is so at approximately room temperature.

Mounted within the lower part of the support tube 5 is a subsidiaryvessel hereinafter referred to as the transfer box I0, The bottom of thetransfer box 10 has an outlet 11 which can be closed by a transfer valve12 controlled from outside the, vessel I by an operating rod 13. Whenthe outlet 11 is closed by the transfer valve 12, helium gas can bepumped from the transfer box I0 by Way of an outlet tube 1 4, but thetransfer valve 12 closes the outlet tube 14 when the outlet 1I isopened. Helium gas is continuously pumped from the condenser 4 by wayof-an outlet tube 15. Liquid helium can be admitted to the transfer boxI0 by way of a vacuum-insulated transfer tube I6 connectedto the DewarS.

Projecting intothe vessel I are three vapour pressure thermometers T1,T2 and T3. The bulb of the thermometer T3 isin the transfer box 10, andthe bulbs of the thermometers4 T1y and T2 are in the condenser 4. Thebulb of the thermometer T2 is comparatively near the bottom of thecondenser 4 and defines the level of liquid helium at whichreplenishment is necessary, and the bulb of the thermometer T1 iscomparatively hear the top of the condenser 4 and defines the level ofliquid helium at which replenishment ceases.

Each of the thermometers T1, T2 and T3 is arranged to supply `an outputelectric signal when the temperature, t1, rz,y or t3, respectively, ofthe bulb rises above a preset temperature, t1', t2', t3 respectively,these preset temper-atures corresponding to immersion in liquid helium.In other words, each of the thermometers T1, T2 and T3 suppliesk anelectric signal when the bulb is in helium gas and. hence t1 lz1 or t2t2 or t3 t3.

Referring now to FIGURE 2 this shows, in very diagrammatic form, thevessel I, the Dewar S and the associated pumping arangements. Therequired pumping is provided. continuously by a vapour-pumping rotarypump 20 which expels helium gas by way of an outlet 21 to a recoverysystem (not shown). A low pressure is thus maintained on a common vacuumline 22.

The outlet tube 15 from the condenser 4 can be connected to the vacuumline 22 by way of normally closed needle valves VCI, VCZ and VCS. Todecrease the pressure in the condenser 4 valves VCI, VCZ and VCS can beopened by energising electromagnetic relays ECI, EC2 and ECSrespectively. The valves VCI, VC2 and, VCS have orifices ofprogressively decreasing sizes, so that the pressure in the condenser 4can be Controlled by the sequential energisation of the appropriaterelays ECI, EC2 and ECS.

The outlet tube 14 from the transfer box I0t can be connected to thevacuum line 22 by way of normally closed needle valves VTI, VTZ and VTS.To decrease the pressure in the transfer box 10 valves VTI, VTZ and VTScan be opened by energisring electromagnetic relays ETI, ETZ and ETSrespectively. The valves VTI, VTZ and VTS have orices of progressivelydecreasing sizes, so that the pressure in the transfer box 10 can becontrolled by the sequential energisation of the appropriate relays ETI,ETZ and ETS.

There is also an outlet tube 23 from the Dewar S which can be connectedto the vacuum line 22 by way of normally closed needle valves VDI, VD2and VDS. To decrease the pressure in the Dewar S, valves VDI, VD2 andVDS can be opened by energising electromagnetic relays EDI, EDZ and EDSrespectively. The valves VDI, VD2 and VDS have orices of progressivelydecreasing sizes, so that the pressure in the Dewar S can be controlledby the sequential energisation of the appropriate relays EDI, E132 andEDS.

The outlet tube I5 from the condenser 4 can also be connected to astorage line 24 by energising an electromagnetic relay EC4 which opens anormally closed valve VC4. The valve VC4 is also bypassed by anon-return valve 25 for safety purposes.

The outlet tube 14 from the transfer box 10 can be connected to thestorage line 24 by energising an electromagnetic relay ET4., which opensa normally closed valve VT4. The valve VT4 is also bypassed by anonreturn, valve 26 for safety purposes.

The outlet tube 23 fromthe Dewar 3 can also be con-` nected tothestorage line 24 byv energising an electromagnetic relay ED'4 which opensa normally closed valve VD4. The valve VD4 isl also bypassed by anonfreturn valve 27 for safety purposes. i

The storage line 24 is connected to a helium storage bag (not shown).This enables operation of the cryopump to be stopped without necessarilyopening it to the atmosphere. Itis desirable to have this featureotherwise it is necessary to ush the cryopump with dry heliumr gasbefore starting operation, as if this is not done iceA is likely to forminthe cryopump and may interfere with the operation, for example byjamming the transfer valve I2. v i

The pressure in the condenser 4 is normally such that the liquid heliumin the condenser 4 is at the temperature at which it is desired tomaintain the surface 2, whilst the pressure in both the transfer box 10and the Dewar S is approximately 1 to 2 centimetres of mercury highercorresponding to a temperature some 0.03vo K. higher. Y Y

Reference will now be made to FIGURE 3 ofthe drawings which shows, verydiagrammatically, the pressure measuring system. Basically thiscomprises a series of interconnected mercury manometers, three arms 30,31 and S2 of which are usedy to obtain'pressure indications from ythecondenser 4, transfer box and Dewar S, respectively, shown in FIGURES Iand 2. There are therefore connections 3S, S4 and S5 from thetops of thearms S0, 31 and S2 to the condenser 4, transfer box I0 and Dewar 3,respectively. The. arrows alongside the arms 30, S1 and 32 indicate thedirection in which the mercury moves in the arm 30, SI or'SZ as thepressure in the condenser 4, transfer box or Dewar 3 respectivelydecreases.

Near the bottom of the arm 30 is in electrical contact 36 projectinginto the mercury, the contact 36 also being connected to a common line37. At successively higher points in the arm 30 are four furtherelectric contacts projecting into the arm 30, these contacts being forconvenience referenced PCI, PC2, PCS and PCI because they correspond tothe position of the top of the mercury for four successively lowervalues Pci, PC2, FC3 and PCI of the instantaneous pressure PC in thelcondenser 4 (FIG- URES 1 and 2). The arrangement is such that anelectric signal is Isupplied to a logic circuit to be describedsubsequently for each of the contacts PCI, etc., which is uncovered. Iffor example the condition is that is to say if the top of the mercury isbetween contacts PC3 and PC2, then two electric signals will be suppliedto the logic circuit, these corresponding to PC PC3 and PC PC4.

The actual arrangement of the contacts PCI, etc., is shown in more`detail in FIGURE 4 of the drawings, to which reference is now made.Mounted within the arm 30 is a stainless steel tube 38 which at itslower end carries the contacts PCI, etc., each projecting beyond thepreceding one. Connections to the contacts P01, etc., are by way ofleads 39.

Referring again to FIGURE 3, near the bottom of the arm 31 is anelectrical contact 40 projecting into the mercury, the contact 40 alsobeing connected to the common line 37. At successively higher points inthe arm 31 are four further electrical contacts projecting into the arm31, these contacts being for convenience referenced PTI, PT2, PT3 andPII-4 because they correspond to the position of the top of the mercuryfor four successively lower values PTI, PT2, PTB and PTI of theinstantaneous pressure PT in the transfer box 10 (FIGURES l and 2). Thearrangement and operation of the contacts PTI, etc. is similar to thatof the contacts PCI, etc.

Near the bottom of the arm 32 is an electrical contact 41 projectinginto the mercury, the contact 41 also being connected to the common line37. At successively higher points in the arm 32 are four furtherelectrical contacts projecting into the arm 32, these contacts being forconvenience references PDI, PD2, PD3 and PDI because they correspond tothe position of the top of the mercury for four successively lowervalues PDI, PD2, PD3 and PDI of the instantaneous pressure PD in theDewar 3 (FIGURES l and 2). The arrangement and operation of the contactsPDI, etc. is similar to that of the contacts P01, etc.

The contacts PCI, etc., and PDI, etc., are movable so that thetemperatures in the condenser 4 and Dewar 3 (FIGURES 1 and 2) may bereadily set at any desired value in the range 3.1" K. to 3.9 K. Thecontacts PII-I, etc. are lixed. The contact PTI is set just below theequi-pressure mercury level, and contact PT2 is set just above thislevel, so that the valve VTI is operated (in a way which will becomeclear subsequently) to maintain the transfer box 10 at the same pressureas the Dewar 3 (FIGURES 1 and 2). The contacts PII-3 and PfDI are setabout 11/2 centimetres higher to achieve the pressure differentialbetween the transfer box 10 and Dewar 3 (FIGURES l and 2) required fortransfer of liquid helium.

The overall operation of the cryopump is controlled by the logic circuitshown in FIGURE 5 of the drawings, to which reference is nowv made. Someof the symbols used in FIGURE 5 will first be explained.

The conventional symbols are used for OR gates, AND gates and NOT gates.

The circuit includes a number of switches which are closed when acertain operation is to be performed. Each such switch is represented bya double lined rectangle enclosing a brief description of the relevantoperation.

Certain conditions sensed by the thermometers and manometers previouslyreferred to result in electric signals being supplied. Each such signalsource is represented by a rectangle enclosing an indication of thecondition which results in a signal being supplied, for example, Pc Pc4The fulfilment of certain conditions is indicated by lamps. Each suchlamp is represented by a rectangle with curved ends enclosing a briefdescription of the relevant condition.

On the fulfilment of certain conditions one or other of theelectromagnetic relays ETI, etc., (FIGURE 2) is er1- ergised. This isrepresented by squares enclosing relay references, and the operation issuch that when a signal is supplied to the input to a relay, that relayis energised and the associated valve is opened.

On the fulfilment of certain other conditions the transfer valve 12(FIGURE 1) changes position, and instead of closing the outlet 11 itcloses the outlet tube 14. This occurs when a signal is supplied to therectangle designated transfer valve.

The operation of the logic circuit will not be described exhaustively,but some parts of the operation will be 4described and from this theremainder of the operation can readily be deduced. In this descriptionreference will also be made to FIGURES l, 2 and 3. I

The part of the operation which controls the pressure in the condenser 4and hence the temperature of the liquid helium in the condenser 4 willrst be considered. As stated above, the manometer serves to compare theinstantaneous pressure PC in the condenser 4 with preset pressure valuesPCI, PC2, etc. in sequence of reducing pressure. At any time when PC PCIeach of the AND gates 51, 52 and 53 is supplying an output signal, sothat the relays ECI, ECZ and ECS are energised and the valves VCI, VC2and VCS are all open. The pressure in the condenser 4 is thereforereduced at the maximum pumping speed until PC PC2 whereupon one of theinput signals to the AND gate 53 ceases, the relay ECI is `de-energised,and the Valve VC1 closes reducing the pumping speed.

Similarly the other valves VC2, etc. close as the pressure is furtherreduced until the pumping speed approximates to the rate of evaporationof helium from the condenser 4.

If the pumping speed is now too low the pressure in the condenser 4 willrise until the ymercury in the arm 30 falls past the next lowest contactand the associated valve is opened to increase the pumping speed. If thepumping speed is now too high the pressure in the condenser 4 will falluntil the mercury in the arm 30 rises past the next highest contact andthe associated valve is closed to decrease the pumping speed. Thisarrangement results in little chattering of the relays and valves.

The valves VCI, VC2 and VC3 are adjusted so that the system normallyoperates with the pressure PC in the condenser 4 between PC2 and FC3.

The part of the operation which controls the pressure in the Dewar 3 andhence the temperature of the liquid helium in the Dewar 3 is similar,with the exception that when PD PD.I, the valve VD4 is opened to allowhelium gas to enter the Dewar 3, so that the correct pressure can bemaintained during transfer.

The part of the operati-on which controls the transfer of liquid heliumautomatically from the Dewar 3 to the condenser 4 will now ybedescribed.

f First it is assumed that the condenser 4 is full, so that the bulbs ofthermometers T1 and T2 are both immersed in liquid helium. This meansthat there is no input signal to the AND gate 54 and that the condenserfull lamp 55 is lit. When the level of the liquid helium has fallen tothe extent that the bulbs of both thermometers T1 and T2 are uncovered,then signals corresponding to tI tI and t2 t2 are supplied to the ANDgate 54 which supplies a signal to the AND gate 56. Also the condenserfull lamp '7 55 is extinguished and the condenser empty lamp 57 is lit.

The AND'gate 56 should now supply an output signal because theconditions should normally be such that signals are already lbeingsupplied over al1 the other inputs. These conditions are as follows.

The condenser pressure should be right, that is to say PC PC1, so thatthe condenser pressure right lamp 58 is lit and a signal supplied to theAND gate 56.

The Dewar pressure should be right, that is t-o say PD PD1, so that theDewar pressure right lamp 59 is lit and a signal supplied to the ANDgate 56.

The system pressure should be right. Clearly there is no point inmaintaining the surface 2 cold if the pressure in the system beingpumped is so high that no useful cryopumping can occur. The systempressure Ps is therefore monitored and so long as it is below somepredetermined value PS', the system pressure right lamp 60 is lit, and asignal supplied to the AND gate 56.

The radiation shield 7 should be cold, in other words there should beliquid nitrogen in the jacket 8. So long as this is the case thetemperature tR of the radiation shield 7 is belowe some predeterminedvalue IR', the radiation shield cold lamp 61 is lit, and a signal issupplied to the AND gate 56.

Lastly the Dewar 3 should contain liquid helium. This is indicated by afurther vapour pressure thermometer T4. So long as the bulb of thisthermometer T4 is immersed, so that t4 t4', the Dewar empty lamp 62 isextinguished, and a signal is supplied to the AND gate 56.

The AND gate 56 will therefore supply a signal to the OR gate 63, whichwill supply a signal to the AND gate 64. The condenser system startswitch Sti and the helium supply system start switch 65 must be closed,so assuming the hold transfer switch 66 has not been closed, the ANDgate 64 supplies a signal to the AND gate 67, and to the AND gates 68and 69. This causes the valves VTZ and VT3 to be opened increasing thepumping speed in the transfer box 10, and so reducing the pressure inthe transfer box 10 below that of the Dewar 3. This results in liquidhelium siphoning from the Dewar 3 by way of the transfer tube 16 intothe transfer box 10. The transfer valve 12 is at this time closing theoutlet 11.

At first the helium entering the transfer box 10 will be in the form ofgas and pumping of the transfer box 10 will therefore continue untilthere is liquid helium in the transfer box 10. The pressure PT in thetransfer box 10 will then fall until when PT PT3, a signal is suppliedto the AND gate 67, and the transfer condition lamp 70 is lit. When thebulb of the thermometer T 3 is immersed in liquid helium, t3 t3', asignal is supplied to the AND gate 67 and the heliumy in transfer boxlamp 71 is lit.

The AND gate 67 then supplies a signal to the OR gate 72 which suppliesa signal to the transfer valve operating relay 73 which causes thetransfer valve 12 to lift off the outlet 11 and close the outlet tube14, so that liquid helium is transferred to the condenser 4, Thetransfer proceeding lamp 74 is also lit.

W'nen the condenser 4 is full, that is when the bulb of the thermometerT1 is immersed in liquid helium, t1 t1', so there is no longer an outputsignal from the AND gate 54. This causes valves VT2 and VTS to be closedso that the pressure in the transfer box 10 rises to that of the Dewar3. This means that PT PT3, the transfer valve 12 closes the outlet 11and the transfer is terminated.

In a particular embodiment of the cryopump described it has been foundthat during the replenishment of the condenser 4 the temperature of theliquid helium in the condenser 4 does not vary from the desiredtemperature by more than i0.02 K.

If the condenser 4 is completely empty, as when starting up, the systemwill in fact operate automatically to fill it with liquid helium andbring it to the desired ternperature. It is however more economical ofliquid helium to use the manual transfer request switch 75 and manual 8transfer proceed switch 76 which bypass many of the normal conditionsand enable helium gas from the Dewar 3 to be pumped through the transferbox 10 and condenser 4 to precool them.

Although a cryopump has been described, it will be appreciated that thearrangement can equally well be used where the condenser 4 does not formpart of a cryopump but is part of some other cryogenic device whichneeds to be kept at a predetermined temperature and to be replenishedwithout causing this temperature to vary appreciably.

I claim:

1. A cryopump comprising a surface arranged to be in contact with thegas within a system to be pumped, a first vessel arranged to contain acryogenic liquid in good thermal contact with said surface so that thesurface is maintained substantially at the temperature of the cryogenicliquid, this temperature being such that there is a net condensation ofgas phase molecules from the system on to said surface, means tomaintain the cryogenic liquid in the first vessel substantially at adesired temperature, a subsidiary Vessel communicating directly with thefirst vessel, a connection through which cryogenic liquid is arranged tobe supplied from a second vessel containing a further quantity of thesame cryogenic liquid to said subsidiary vessel when the cryogenicliquid in the first vessel needs replenishing, and means toallowcryogenic liquid to pass from said subsidiary vessel into the firstvessel only when the temperature of the cryogenic liquid in saidsubsidiary Vessel is substantially equal to said desired temperature.

2. A cryopump in accordance with claim 1 wherein said means to maintainthe cryogenic liquid in the first vessel substantially at a desiredtemperature comprises a pumping arrangement which operates to maintainthe pressure in the first vessel substantially at a predetermined value.

3. A cryopump in accordance with claim 2 wherein said means to allowcryogenic liquid to pass from said subsidiary vessel into the firstvessel comprises an outlet from said subsidiary vessel into the firstvessel, a valve which normally closes said outlet, means to sense whenthe cryogenic liquid in the first vessel falls below a firstpredetermined level and when this occurs to cause said pumpingarrangement to reduce the pressure in said subsidiary vessel so thatcryogenic fluid passes through said connection from the second vessel tosaid subsidiary vessel, the cryogenic iiuid normally being gaseousinitially but becoming liquid as pumping proceeds, means to sense whenthe cryogenic liquid in said subsidiary vessel rises above apredetermined level, means thereupon to open said valve so thatcryogenic liquid passes from said subsidiary vessel into the firstvessel, means to sense when the cryogenic liquid in the first vesselrises above a second predetermined level higher than the firstpredetermined level, and means thereupon to stop the replenishment.

4. A cryopump in accordance with claim 3 wherein the cryogenic liquid isliquid helium.

5. A cryopump in accordance with claim 4 wherein the gas within saidsystem to be pumped is predominantly hydrogen and wherein said pumpingarrangement which maintains the liquid helium in the first vesselsubstantially at a desired temperature operates to maintain the pressurein the first vessel substantially at a predetermined value lying withinthe range 210 to 580 millimetres of mercury.

6. A cryogenic device comprising a first vessel arranged to contain acryogenic liquid, a pumping arrangement which operates to maintain thepressure in the first vessel substantially at a predetermined value sothat the cryogenic liquid in the first vessel is maintainedsubstantially at a desired temperature, a subsidiary vesselcommunicating directly with the first vessel, a connection through whichcryogenic liquid is arranged to be supplied from a second vesselcontaining a further quantity of the same cryogenic liquid to saidsubsidiary vessel when the cryogenic liquid in the first vessel needsreplenishing, and means to allow cryogenic liquid to pass from saidsubsidiary vessel into the rst vessel only when the temperature of thecryogenic liquid in said subsidiary vessel is substantially equal tosaid desired temperature, said means comprising an outlet from saidsubsidiary vessel into the first vessel, a valve which normally closessaid outlet, means to sense when the cryogenic liquid in the firstvessel falls below a rst predetermined level and when this occurs tocause said pumping arrangement to reduce the pressure in said subsidiaryvessel so that cryogenic uid passes through said connection from thesecond vessel to said subsidiary vessel, the cryogenic fluid normallybeing gaseous initially but becoming liquid as pumping proceeds, meansto sense when the cryogenic liquid in said subsidiary vessel [risesabove a predetermined level, means thereupon to open said valve so thatcryo- 10 genie liquid passes from said subsidiary vessel into the rstvessel, means to sense when the cryogenic liquid in the rst vessel risesabove a predetermined level, and means thereupon for stopping thereplenishment.

7. A cryogenic device in accordance with claim 6 wherein the cryogenicliquid is liquid helium.

References Cited

1. A CRYOPUMP COMPRISING A SURFACE ARRANGED TO BE IN CONTACT WITH THEGAS WITHIN A SYSTEM TO BE PUMPED, A FIRST VESSEL ARRANGED TO CONTAIN ACRYOGENIC LIQUID IN GOOD THERMAL CONTACT WITH SAID SURFACE SO THAT THESURFACE IS MAINTAINED SUBSTANTIALLY AT THE TEMPERATURE OF THE CRYOGENICLIQUID, THIS TEMPERATURE BEING SUCH THAT THERE IS A NET CONDENSATION OFGAS PHASE MOLECULES FROM THE SYSTEM ON TO SAID SURFACE, MEANS TOMAINTAIN THE CRYOGENIC LIQUID IN THE FIRST VESSEL SUBSTANTIALLY AT ADESIRED TEMPERATURE, A SUBSIDIARY VESSEL COMMUNICATING DIRECTLY WITH THEFIRST VESSEL, A CONNECTION THROUGH WHICH CYYOGENIC LIQUID IS ARRANGED TOBE SUPPLIED FROM A SECOND VESSEL CONTAINING A FURTHER QUANTITY OF THESAME CYROGENIC LIQUID TO SAID SUBSIDIARY VESSEL WHEN THE CRYOGENICLIQUID IN THE FIRST VESSEL NEEDS REPLENISHING, AND MEANS TO ALLOWCRYOGENIC LIQUID TO PASS FROM SAID SUBSIDIARY VESSEL INTO THE FIRSTVESSEL ONLY WHEN THE TEMPERATURE OF THE CRYOGENIC LIQUID IN SAIDSUBSIDIARY VESSEL IS SUBSTANTIALLY EQUAL TO SAID DESIRED TEMPERATURE.